From the tropics to the arctic, both climate and weather have powerful impacts, both direct and indirect, on human life. While people adapt to the conditions in which they live, and human physiology can handle substantial variation in weather, there are limits.

Extremes of both heat and cold can cause potentially fatal illnesses, e.g. heat stress or hypothermia, as well as increasing death rates from heart and respiratory diseases.

In cities, stagnant weather conditions can trap both warm air and air pollutants -- leading to smog episodes with significant health impacts.

These effects can be significant. Abnormally high temperatures in Europe in the summer of 2003 were associated with at least 27,000 more deaths than the equivalent period in previous years1 .

Other weather extremes, such as heavy rains, floods, and hurricanes, also have severe impacts on health. Approximately 600,000 deaths occurred world-wide as a result of weather-related natural disasters in the 1990s; and some 95% of these were in poor countries. Some examples:

In October 1999, a cyclone in Orissa, India, caused 10,000 deaths. The total number of people affected was estimated at 10-15 million;

In December 1999, floods in and around Caracas, Venezuela, killed approximately 30,000 people, many in shanty towns on exposed slopes.

In addition to changing weather patterns, climatic conditions affect diseases transmitted through water, and via vectors such as mosquitoes. Climate-sensitive diseases are among the largest global killers. Diarrhoea, malaria and protein-energy malnutrition alone caused more than 3.3 million deaths globally in 2002, with 29 % of these deaths occurring in the Region of Africa.

THE SETTING: GLOBAL WARMING

About two thirds of solar energy reaching Earth is absorbed by, and heats, the Earth's surface. The heat radiates back to the atmosphere, where some of it is trapped by greenhouse gases, such as carbon dioxide. Without this 'greenhouse effect' the average surface temperature would make the planet uninhabitable for human populations.

Human activities, particularly burning of fossil fuels, have released over the last 50 years, sufficient quantities of CO2 and other greenhouse gases to affect the global climate. The atmospheric concentration of carbon dioxide has increased by more than 30% since pre-industrial times, trapping more heat in the lower atmosphere.

According to the Third Assessment Report (2001) of the Intergovernmental Panel on Climate Change (IPCC), some effects include:

The global average surface temperature has increased by 0.6° + 0.2° C over the last century;

Globally, 1998 was the warmest year and the 1990s was the warmest decade on record;

Many areas have experienced increases in rainfall, particularly mid to high latitude countries;

In some regions, such as parts of Asia and Africa, the frequency and intensity of droughts have increased in recent decades;

Episodes of El Niño have been more frequent, persistent and intense since the mid-1970s compared with the previous 100 years.

Global emissions of carbon dioxide are still increasing. Estimates of future population growth and energy use are used as inputs to global climate models, in order to project future climate change. Reviewing outputs from a range of such models, the IPCC has made the following predictions for the next century:

Global mean surface temperature will rise by 1.4°-5.8° C. Warming will be greatest over land areas, and at high latitudes;

The projected rate of warming is greater than anything humans have experienced in the last 10,000 years;

The frequency of weather extremes is likely to change leading to an increased risk of floods and drought. There will be fewer cold spells but more heat waves;

The frequency and intensity of El Niño may be affected;

Global mean sea level is projected to rise by 9--88 cm by the year 2100.

Many countries are working to reduce greenhouse gas emissions under the United Nations Framework Convention on Climate Change. Unfortunately, current international agreements are not sufficient to prevent the world facing significant changes in climate and a rise in sea levels.

THE EFFECTS OF CLIMATE CHANGE ON HEALTH

To a large extent, public health depends on safe drinking water, sufficient food, secure shelter, and good social conditions. A changing climate is likely to affect all of these conditions. Reviews of the likely impacts of climate change by the IPCC suggest that a warming climate is likely to bring some localized benefits, such as decreased winter deaths in temperate climates, and increases in food production in some, particularly high latitude, regions. Public health services and high living standards would protect some populations from specific impacts; for example it is unlikely that climate change would cause malaria to become re-established in northern Europe or North America. Overall, however, the health effects of a rapidly changing climate are likely to be overwhelmingly negative, particularly in the poorest communities, which have contributed least to greenhouse gas emissions. Some of the health effects include:

Increasing frequencies of heatwaves: recent analyses show that human-induced climate change significantly increased the likelihood of the European summer heatwave of 2003.

More variable precipitation patterns are likely to compromise the supply of freshwater, increasing risks of water-borne disease.

Rising temperatures and variable precipitation are likely to decrease the production of staple foods in many of the poorest regions, increasing risks of malnutrition.

Rising sea levels increase the risk of coastal flooding, and may necessitate population displacement. More than half of the world's population now lives within 60km of the sea. Some of the most vulnerable regions are the Nile delta in Egypt, the Ganges-Brahmaputra delta in Bangladesh, and many small islands, such as the Maldives, the Marshall Islands and Tuvalu.

Changes in climate are likely to lengthen the transmission seasons of important vector-borne diseases, and to alter their geographic range, potentially bringing them to regions which lack either population immunity or a strong public health infrastructure.

Measurement of health effects from climate change can only be very approximate. Nevertheless, a WHO quantitative assessment, taking into account only a subset of the possible health impacts, concluded that the effects of the climate change that has occurred since the mid-1970s may have caused over 150,000 deaths in 2000. It also concluded that these impacts are likely to increase in the future.

WHO'S RESPONSE

WHO co-ordinates reviews of the scientific evidence on the links between climate, climate change and health, including supporting the IPCC assessment process. Based on these assessments, WHO considers that rapid climate change poses substantial risks to human health, particularly among the poorest populations. The organization therefore supports actions to reduce human influence on the global climate.

Carefully planned mitigation policies can also bring direct health benefits. For example, well-designed urban transport systems can reduce greenhouse gas emissions, while simultaneously reducing the major health impacts of urban air pollution and physical inactivity. Housing with efficient insulation can cut energy consumption and associated greenhouse gas emissions, reduce deaths from both cold and heat, and in poor countries, reduce the need for burning of biomass fuels and the impacts of indoor air pollution.

WHO also recognizes that, given past emissions of greenhouse gases, the world will continue to be faced with a warming and more variable climate for at least several decades. WHO's work in supporting programmes to combat infectious disease, improve water and sanitation services and respond to natural disasters helps to reduce health vulnerability to future climate change. The organization also works directly to build capacity to adapt to climate change. This includes workshops in the most vulnerable countries to raise awareness of the health implications of climate change and related weather patterns, and to support intersectoral policies to reduce health vulnerability now. Such activities aim at improving health conditions today, while simultaneously laying the ground for more adaptation measures to climate change in the future.

(1) This figure is based only on data from selected cities across Europe; it will be revised as more complete and comparable data from across Europe becomes available.